Rotary rock bits are used to bore earth formations in the drilling of oil and gas wells, geothermal steam wells, and the like. Typical modern rock bits have a main body which is a welded assembly of several (usually three) mating segments or legs. An upper end of the welded legs is machined to form threads enabling attachment of the bit to the lower end of a drill pipe used in conventional rotary drilling. Drilling mud is circulated through the drill pipe and passages in the bit body to cool the bit, and to flush cuttings to the surface through an annulus between the drill pipe and well bore above the bit.
A rotatable cutter is mounted at the lower end of each leg, and the cutters are usually conical in shape and hence called cones. Depending on the type of formation to be drilled by the bit, the cones may have integrally formed teeth, or may be fitted with hard inserts of sintered tungsten carbide or the like. The cone interior has a cylindrical bore which receives a bearing shaft or pin extending from the associated bit leg. The cone is secured against axial movement off the pin by a locking means such as a set of balls fitted into a mating race defined by aligned annular grooves in the cone and pin. These structural features are conventional, and are well known in the art.
Two fundamental objectives in rock-bit design are to provide bits which will drill an accurate hole of controlled diameter, and which will drill the greatest possible depth of earth formation at an acceptable penetration rate before replacement is required due to bit wear. Hole diameter accuracy must be maintained to avoid pinching and time-consuming reaming when the next bit is moved into the hole, and also to insure that the hole is of proper size to receive casing pipe and the like. Bit life and drilling performance are of economic importance because rotary drilling rigs are expensive to staff and maintain, and many hours of unproductive down time are required to pull a worn bit and insert a replacement.
Rock bits operate under heavy loads, and are subjected to high temperatures and an abrasive environment. These tools are accordingly made of high-strength alloys and may incorporate pressure-compensated lubrication systems. Particular attention is given to the bearing structures used to support the cones on the bit-leg pins. Anti-friction bearings are used in some bit styles, but many modern bits intended for heavy service use a journal bearing to transmit loads from the rotating cutter cones to the bit body.
An overall goal in rock-bit design is to provide subsystems (bearings, lubrication, seals, etc.) which will outlast the cutting surfaces of the cones. Achievement of this objective insures that ultimate bit life will be determined by wear of the cutting surfaces, rather than by failure of a subsystem which requires premature replacement of a bit with cutting surfaces capable of further drilling.
In conventional bits, each bit-body segment or leg and bearing pin is a unitary integral body of high-strength steel which is forged and subsequently machined to final shape. Lubrication passages and openings for drilling -- mud jets and the like are machined in the legs, and the cones are installed on the legs prior to final assembly (welding of the legs) because the cones are closely meshed and cannot be mounted after the legs are assembled. The legs are precisely mated to insure that the final bit will have the proper gage (hole size) diameter, and the legs are then welded to form the bit body.
This invention is directed to a non-integral two-piece leg wherein the bearing pin is separately machined and then welded to a lower part of the leg structure. The broad concept of a two-piece leg is known, and welded bearing pins and related arrangements are shown in U.S. Pat. Nos. 1,874,065, 2,039,551, 2,058,155, 2,321,484, 2,329,751 and 2,654,577. The welded surfaces in these prior-art designs, however, were formed by conventional arc or gas welding methods which have low penetration, and which leave a relatively large heat-affected zone in the metal adjacent the weld. This in turn results in warping and a detrimental effect on the properties of heat-treated alloys, and modern bits are accordingly made with integrally forged legs and bearing pins.
The problems encountered in prior-art welded bits are substantially overcome by using electron-beam welding methods to attach a separately machined and heat-treated bearing pin to the associated bit leg. As explained in greater detail below, bits made according to the invention can be produced at lower cost without sacrifice of dimensional accuracy or mechanical strength, and significant energy savings arise from the ability to limit certain heat-treating processes to the bearing pin alone. Electron-beam welding for assembly of other portions of rock bits is known, and is discussed in U.S. Pat. Nos. 3,850,256, 3,907,191, and 3,987,859.